Method for molding ceramic powders using a water-based gel casting process

ABSTRACT

A method for molding ceramic powders comprises forming a slurry mixture including ceramic powder, a dispersant, and a monomer solution. The monomer solution includes at least one monofunctional monomer and at least one difunctional monomer, a free-radical initiator, and a aqueous solvent. The slurry mixture is transferred to a mold, and the mold containing the slurry mixture is heated to polymerize and crosslink the monomer and form a firm polymer-solvent gel matrix. The solid product may be removed from the mold and heated to first remove the solvent and subsequently remove the polymer, whereafter the product may be sintered.

The U.S. government has rights in this invention pursuant to blanketpurchase agreement No. DE-AB01-87GC20701.M000.

This is a divisional of co-pending application Ser. No. 207,813 filed onJun. 17, 1988, now U.S. Pat. No. 5,028,362 which is a continuation inpart of a previously filed co-pending application Ser. No. 158,485 filedFeb. 22, 1988, now U.S. Pat. No. 4,894,194 and relates to a method formolding ceramic powders. More particularly, the present inventionrelates to a method for molding ceramic powders wherein the ceramicpowders are added to a monomer solution to form a slurry mixture whichis formed into a solid, shaped product. The method is particularlyadaptable for forming complex shaped bodies from the ceramic powders.

BACKGROUND OF THE INVENTION

Methods for forming ceramic powders into complex shapes are desirable inmany areas of technology. For example, such methods are required forproducing advanced, high-temperature structural parts such as heatengine components, recuperators and the like from ceramic powders.Generally, two methods are presently known for forming ceramic powdersinto complex or intricately shaped parts. Specifically, one methodcomprises machining a green blank to the desired shape. However, thismethod has significant drawbacks in that the machining is timeconsuming, expensive, and in a practical sense, inapplicable to somecomplex or varied cross-sectional shapes, for example, turbine rotors. Asecond method for forming ceramic powders into complex or intricatleyshaped parts comprises injection molding a composition which comprisesthe ceramic powder and a polymeric and/or wax-like binder as a vehiclefor the ceramic powder.

For example, the Strivens U.S. Pat. No. 2,939,199 discloses a method offorming articles from ceramic powders wherein the ceramic powders aremixed with a vehicle comprising a thermosetting material and aplasticizer, and the resultant mixture is injection molded into a moldof a desired shape and heated to cure the thermosetting component. Thevehicle is then removed from the molded part by low pressuredistillation or by solvent extraction. A similar method is disclosed inthe Kingery et al U.S. Pat. No. 3,351,688 wherein the ceramic powder ismixed with a binder such as paraffin at a temperature where the binderis liquid, and the resulting mixture is cast into a mold of the desiredshape. The binder is permitted to solidify so that a green piece isformed having a uniform density. Use of a paraffin wax binder formolding ceramic powders into desired shapes is also disclosed in theCurry U.S. Pat. No. 4,011,291 and the Ohnsorg U.S. Pat. No. 4,144,207.The Rivers U.S. Pat. No. 4,113,480 and the Wiech, Jr. U.S. Pat. No.4,197,118 disclose additional methods for molding parts from metalpowders by mixing the powders with binder materials and injectionmolding the resultant mixtures. Additional methods of interest whichemploy binder materials are also disclosed in the Huther et al U.S. Pat.No. 4,478,790 and the Kato U.S. Pat. No. 4,460,527.

The aforementioned injection molding techniques using various bindermaterials also have significant drawbacks. Generally, the binder removaltimes are unacceptably long, being up to a week or more in someinstances, and binder removal often creates cracks or warpage in themolded parts. Additionally, after binder removal, the strength of themolded parts is relatively low whereby increased breakage of the partsoccurs during subsequent handling. It is also difficult to providemolded parts having a large cross section, for example, parts greaterthan one inch in cross section, or having widely varying cross sections,that is, with both thick and thin regions, using the injection moldingtechniques.

Thus, the presently known methods for forming complex and intricatelyshaped parts from ceramic powders are disadvantageous in variousrespects. Moreover, a need exists for a method for molding ceramicpowders into complex and intricately shaped parts, which methodovercomes the disadvantages of the known techniques.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amethod for molding ceramic powders into solid, shaped products. It is arelated object of the present invention to provide a method for moldingceramic powders into complex and intricately shaped parts. It is anadditional object of the invention to provide a method for moldingceramic powders into parts of large and/or variable cross sections. Itis a further object of the invention to provide a method for moldingceramic powders into solid, shaped products using a binder vehicle,wherein the time necessary for binder removal is reduced.

These and additional objects are provided by the method for moldingceramic powders according to the present invention. Generally, themethod of the present invention relates to the molding of ceramicpowders into green products wherein a monomer solution is used as abinder vehicle and the controlled thermal polymerization of the monomerin solution serves as a setting mechanism. More specifically, the methodaccording to the present invention comprises forming a slurry mixtureincluding ceramic powder, a dispersant for the ceramic powder, and amonomer solution. The monomer solution includes at least onemonofunctional monomer, at least one difunctional monomer, afree-radical initiator, and an aqueous solvent. The slurry mixture istransferred to a mold, and the mold containing the slurry mixture isheated at a temperature and for a time sufficient for the monomer topolymerize and crosslink to form a firm polymer-water gel matrix. Theresultant green product is of exceptionally high strength and may bedried to remove the water in a relatively short time, for example, inseveral hours. After drying, the product may be further heated to removethe polymer and may subsequently be fired to sinter the product to ahigh density.

These and additional objects and advantages will be more fullyunderstood in view of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a rheogram of RC-152 slurry in water and in D-premix.

FIG. 2 is a graph of the drying rate of green parts at 42° C. and 79° C.

DETAILED DESCRIPTION

The present invention provides a method for molding ceramic powders intosolid, shaped products. The solid, shaped products which result are inthe green state whereby they may be further heated to remove the waterand polymer components and then sintered at high temperatures to providehigh density products. The method of the present invention isparticularly suited for forming solid parts of complex or intricateshapes and/or of large or varied cross sections. Ceramic powderssuitable for use in the methods of the present invention include, butare not limited to, alumina, fused silica, magnesia, zirconia, spinels,mullite, glass frits, tungsten carbide, silicon carbide, boron nitride,silicon nitride, and mixtures thereof.

In accordance with an important feature of the method of the presentinvention, the ceramic powder is mixed with dispersant for the powderand a monomer solution to form a slurry mixture. The monomer solutionprovides a low viscosity vehicle for the ceramic powder. Additionally,when heated, the monomer solution polymerizes and crosslinks to form afirm, strong polymer-solvent gel matrix. The gel matrix immobilizes theceramic powder in the desired shape of the mold in which the slurrymixture is heated. The resultant "green" product exhibits exceptionallyhigh strength.

Various dispersants for ceramic powders are known in the art and areappropriate for use in the present invention. Care should be exercised,however, is order to select a dispersant which does not interact withthe components of the monomer solution, particularly the initiator orthe solvent. A particular dispersant may be evaluated for suitabilitywith a particular ceramic powder and a particular monomer solution bymixing small amounts of the respective components and judging the flowproperties of the resultant mixture, whether the resultant mixtureexhibits a notable yield point, and/or whether the mixture is dilatant.Preferred dispersants in water, include acrylic and methacrylic acidsalts. Generally, the dispersant is used in a small amount, by volume,as compared with the amount, by volume, of the ceramic powder includedin the mixture.

The monomer solution which is mixed with the ceramic powder and thedispersant to form the slurry mixture includes at least onemonofunctional monomer, at least one difunctional monomer, afree-radical initiator compound and an aqueous solvent. Generally, amonofunctional monomer includes one functional group such as a vinyl orallyl group and a difunctional monomer includes two. In accordance witha preferred embodiment of the present invention, the monomer solutionincludes at least one monofunctional acrylamide monomer and at least onedifunctional acrylamide monomer. Generally, the amount of monomerincluded in the monomer solution determines the degree of hardness ofthe resulting solid, shaped product. Generally, an exceptionally hardgreen product can be formed using no greater than about 20 volumepercent of monomers in the monomer solution, and in a preferredembodiment, the monomer solution comprises from about 5 to about 20volume percent monomer.

The use of an aqueous solvent is critical in this process therefore itis necessary to choose a monomer system that is soluble in water at lowtemperatures but polymerizes at increased temperatures.

The monomer solution further comprises a free-radical intiator compoundfor initiating the polymerization and crosslinking of the monomer whenthe slurry mixture is heated. Various thermally activated free-radicalinitiator compounds are known in the polymer art and are suitable foruse in the method of the present invention. Preferred free radicalinitiator compounds include ammonium persulfate and potassiumpersulfate. The free-radical initiator is generally inactive at ambienttemperatures so that the shelf-life of the monomer solution isrelatively long. However, once the slurry mixture containing the monomersolution is heated, the reaction rate of the initiator compound isrelatively high whereby polymerization and crosslinking of the monomersis easily and quickly achieved. The amount of initiator included in themonomer solutions is generally small as compared with the amount ofmonomer in accordance with conventional polymerization methods.

The ceramic powder, the dispersant and the monomer solution may becombined in any suitable manner. In a preferred embodiment, the slurrymixture is formed by dissolving the dispersant in the monomer solutionand then adding the ceramic powder to the solution. The resultant slurrymixture is then transferred to a mold, and the mold containing theslurry mixture is heated at a temperature and for a time sufficient forthe monomer to polymerize and crosslink to form a firm polymer-solventgel matrix. Although the exact temperature at which polymerization andcrosslinking occurs depends on the particular free-radical initiatorcompound and the particular multifunctional monomers which are includedin the monomer solution, generally the temperature should be greaterthan about 25° C., and preferably in the range of about 25° C. to 80° C.Similarly, the time necessary to form a firm polymer-solvent gel matrixis dependent on the particular monomer, solvent and free-radicalinitiator compound. Generally, the mold containing the slurry mixtureshould be heated for at least 10 minutes and preferably is heated for aperiod of from about 10 to about 120 minutes in order to polymerize andcrosslink the monomer and form the firm polymer-solvent gel matrix.After heating, the resultant shaped, solid product may be cooled toambient temperature and removed from the mold. The product is in a wet,green condition in that it contains solvent and is in the unsinteredform. Wet, green products have exhibited extreme strength and toughness.

The wet, green product may subsequently be heated in order tosubstantially remove the water and provide a dry product. Although thespecific temperature and time necessary for drying the product dependson the specific ceramic powder and monomer solution employed, generallydrying may be effected by heating at a temperature greater than about30° C., preferably at approximately 40° to 80° C., in an oven for aperiod greater than about 1 h, preferably for a period of from about 1to about 6 hours. Thus, the drying time in the method of the inventionis substantially reduced as compared with known methods. Additionally,the polymer may be substantially removed from the product by furtherheating at a higher temperature, for example, greater than about 300° C.Finally, the solid, shaped product may be sintered to form a highdensity body. Sintering temperatures for various ceramic powders arewell known in the art. Alternatively, substantial removal of the polymermay be accomplished as a low-temperature step of the sintering process.

While injection molding is preferred for use in the method of thepresent invention, other molding techniques, including extrusionmolding, may also be employed. Moreover, any conventional additivesknown in the ceramic processing arts, for example, mold release agents,may be included in the slurry mixtures for their known functions.

The following Example further demonstrates the method of the presentinvention.

EXAMPLE

Acrylamide (AM) and N,N'-methylenebisacrylamide (MBAM) were themonofunctional and difunctional monomers, respectively, in this system.Total monomer concentrations of 5 percent, 10 percent, 14.6 percent,18.2 percent, respectively, were the standard premix solutions used inall the detailed investigations.

Water with a concentration varying from 81.8 to 95 percent was the onlysolvent. No co-solvent was necessary. The initiators were persulfates;potassium persulfate with a solubility of about 5 percent and ammoniumpersulfate, with solubility over 60 percent in water, were both used.The latter initiator was utilized preferentially in the experiments.

Table 1 shows the range of monomer concentrations used. Theconcentration of monofunctional AM varied from 4.8 to 18.0 weightpercent and the concentration of difunctional MBAM varied from 0.2 to0.6 weight percent for a total concentration of 5.0 to 18.2 weightpercent.

Gelation can be accomplished under extremely varied conditions dependingon whether an initiator alone was added, or an initiator and a catalyst.The catalyst used here is N,N,N¹,N¹ -tetramethylene diamine (TEMED)available from Malinkrodt, Inc., Paris, KY. The rate of gelation dependson how much TEMED is added at a given temperature.

The following conditions for gelation of the premix solutions wereinvestigated:

a. The premix solution without initiator did not gel even underautoclave conditions (130° C.). The acrylamide premix in essence, hasinfinite shelf-life even at elevated temperatures.

b. With the initiator (NH₄)₂ S₂ O₈ at concentrations of 0.5 percent orless, gelation occurred in a water bath at 60° C. or 80° C. within 5minutes; likewise, gelation occurred in 10 to 40 seconds in a microwaveoven depending on the power output.

c. The addition of the catalyst, TEMED, in small amount less than 0.1percent of the total solution further extends the flexibility in thegelation conditions. Gels were formed at room temperature in minutesdepending on how much catalyst was added.

The gelation process was highly exothermic. The gels were transparentand their strength increased with the total monomer concentration. Thegels formed homogeneously throughout the solution with no observablegelation front. There was no phase separation. The gel, whether soft atlow monomer concentration or firm at higher monomer concentration,retained its consistency without a solvent separating out.

The slurry was prepared using Darvan C, an ammonium polymethacrylate,available from R. T. Vanderbilt Co., Greenwich, CT, and P-35, apolyacrylamide, available from American Cyanamid Co., Wayne, NJ, twodispersants common to to ceramic processing. Up to 58 volume percentalumina (RC/52DBM, Reynolds Chem. Co., Bauxite, AR) could be mixed withthe A, B, C, and D premixes to produce freely flowing slurries.

The gelcasting batch compositions were all made in the following ratio:9 ml of premix solution to 1 gm of dispersant to 50 gm of RC-152alumina, equivalent to a 55 volume percent solids loading. FIG. 1 showsthe rheograms of a slurry made with premix D compared to an identicalslurry where water replaced the premix solution. The rheograms for A, B,C, lie between the two. There is little discernable difference among therheograms.

To gel the slurry the calculated amount of the initiator (NH₄)₂ S₂ O₈,was added to a measured quantity of the A, B, C, and D slurries. Theslurries were thoroughly mixed, sonicated and deaired in a paddle mixer.Since all the slurries contained 55 volume percent solids, the polymerrepresents only 0.9 percent, 1.8 percent, 2.6 percent, 3.3 percent ofthe weight of alumina in A, B, C, and D green parts, respectively.

The same flexibility with gelation conditions of the premix solution isapplicable to the slurry. With the initiator alone, the slurries weregelled successfully in waterbath at 60° C. or 80° C. within 30 minutes.

With the addition of the catalyst, TEMED, the gelling occurred at roomtemperature (22°±1° C.) in 10 minutes to an hour depending on both theamount of catalyst and the total monomer concentration. The higher theconcentration, the shorter the gelation time. The quantity of thecatalyst required to gel the slurry was two to three times that requiredfor the premix solution. This is not unexpected as some of the catalystmay have been adsorbed onto the alumina surface. In all these cases,there was no phase separation. The slurry gelled to a single green parteven in complex shapes like a turbine rotor.

The slurries were gelled in glass molds 15.9 mm in diameter by 19.1 mmhigh at room temperature by adding both initiator and catalyst to theslurry. The gelation was complete in all cases in less than an hour. Nomold release was necessary.

Two sets of formed parts of A, B, C, and D were selected for dryingtests. The wet formed parts were smooth, without visible distortions andvery resilient; A was most flexible, D was most rigid. These sampleswere dried in ovens set at 42°±1° C. and 79°±1° C. The weight loss withtime was measured and is plotted in FIG. 2.

At 42° C., there is little difference in the rate of water loss from thegelled part at the four polymer concentrations; 5 percent, 10 percent,14.6 percent, and 18.2 percent. This implies that at this lowtemperature, the water loss is limited by evaporation from the surface.At 79° C., on the other hand, the rate of water loss is highest for thelowest concentration of polymer and decrease with increasing polymercontent. Water loss here is controlled by diffusion through the polymergel.

As the samples dried, the top layers separated off as a thin (<0.5 mm)disc which left a clean smooth surface on the green part. The crackedlayer corresponded to the surface of the slurry exposed to theatmosphere during gelation. The cracking is probably due to theinhibition of the polymerization process by oxygen in the air.Typically, molds would completely enclose the slurries, and if airtightwould eliminate these cracks. The gelation process had been done in aninert atmosphere, such as N₂, which prevented the cracking.

                  TABLE I                                                         ______________________________________                                        Acrylamide premix solutions                                                   Sample   Monomer*     Crosslinking**                                                                             H.sub.2 O                                  Number   AM (%)       agent MBAM(%)                                                                              (%)                                        ______________________________________                                        137      10.0         1.0          89.0                                       138       8.0         1.2          90.8                                       139       4.0         1.6          94.4                                       140      14.0         0.6          85.4                                       152-A     4.8         0.2          95.0                                       153-B     9.6         0.4          90.0                                       154-C    14.0         0.6          85.4                                       155-D    18.0         0.2          81.8                                       ______________________________________                                         *AM-Acrylamide from BRL, Bethesda Research Labs, Gaithersburg, MD.            **MBAM = N,N' methalenebisacrylamide from BRL, Bethesda Research Labs,        Gaithersburg, MD.                                                        

The preceding example is set forth to illustrate specific embodiments ofthe invention and is not intended to limit the scope of the methods orcompositions of the present invention. Additional embodiments andadvantages within the scope of the claimed invention will be apparent toone of ordinary skill in the art.

The acrylamide gelcasting technique represents a major advance inceramic processing. It is a generic system and is not dependent on theparticular composition of the ceramic powder. Also, it represents a verysmall departure from traditional ceramic processing in its application.Slurries can be made with traditional processing equipment such as highspeed dispersers or ball mills. The acrylamide and bisacrylamidemonomers simply substitute for polyvinyl alcohol or other binders andactually reduce the relative viscosity of the system as compared to thepolyvinyl alcohol-containing systems. Molds for gelcasting can be madeof virtually any material including glass, metals, plastics, or waxes.The gelation conditions can be varied from several minutes to hours andfrom ambient to elevated temperatures. The consistency of the as-formed,wet parts can likewise be varied from soft to firm depending on therequirements of a particular process. Thus the aqueous gelcasting systemrepresents a very flexible engineering process for forming ceramics.

We claim:
 1. A ceramic product made by the process comprising:(a)forming a slurry mixture comprising ceramic powder, a dispersant forsaid powder, and a monomer solution including at least one water solublemonofunctional monomer and at least one water soluble difunctionalmonomer, the functional group of said monofunctional and difunctionalmonomers selected from vinyl and allyl groups, a free radical initiatorcompound and an aqueous solvent; (b) transferring said slurry mixture toa mold; and (c) heating said mold containing said slurry mixture at atemperature and for a time sufficient for said monomers to polymerizeand crosslink to form a firm polymer-solvent gel matrix, whereby saidslurry mixture is formed into a solid shaped product.
 2. A ceramicproduct made by the process comprising:forming a slurry mixturecomprising ceramic powder, a monomer solution including at least onemonofunctional monomer and at least one difunctional monomer whereinsaid monofunctional monomer is acrylamide and said difunctional monomeris N,N'-methylenebisacrylamide, a persulfate free-radical initiator, adispersant selected from the group consisting of acrylic and methacrylicacid salts, and an aqueous solvent; transferring said slurry mixtureinto a mold; and heating said mold containing said slurry mixture at atemperature and for a time sufficient for said monofunctional and saiddifunctional monomers to polymerize and crosslink to form a firmpolymer-solvent gel matrix, whereby said slurry mixture is formed into asolid, shaped product.
 3. A ceramic product made by the processcomprising:preparing a slurry mixture comprising ceramic powder, adispersant for said powder, and a monomer solution including at leastacrylamide, N,N'-methylenebisacrylamide, a persulfate and an aqueoussolvent; transferring said slurry mixture to a mold; maintaining saidmold containing said slurry mixture in an inert atmosphere at atemperature and for a time sufficient for said monomers of said monomersolution to polymerize and crosslink to form a firm polymer-solvent gelmatrix, whereby said slurry mixture is fortified into a solid, shapedproduct; removing said solid, shaped product from said mold; heatingsaid removed solid, shaped product at a temperature greater than about30 degrees C. for a period greater than about 1 hour to substantiallyremove said aqueous solvent from said product; further heating saidsolid, shaped product at a temperature greater than about 300 degrees C.and for a time sufficient to substantially remove said polymer from saidshaped product; and sintering said shaped product at a temperaturesufficient to form said high density shaped ceramic object.